1. Field of the Invention
The present invention relates to electrolytic cells wherein alkali metal chlorides, particularly NaCl aqueous solutions, are electrolyzed between an anode and a cathode and wherein a non-rigid, substantially hydraulically, non-impermeable, polymeric plastic film type membrane is disposed between the anode and the cathode.
2. Description of the Prior Art
The consensus of the prior art teachings regarding the location of non-rigid, hydraulically-impermeable, polymeric plastic film-type membranes with respect to adjacent, but spaced, anodes and cathodes is that the membrane should be disposed away from the cathode, being disposed either against the anode or off the surface of the anode while still disposed away from the cathode. This is particularly true for those membranes characterized as being permionic, permselective, or ion-exchange type membranes. The reasons given for so locating the membrane primarily deal with preventing large cell voltage increases taught as occurring when the membrane is located on the cathode. See for example: Seko et al., U.S. Pat. No. 4,108,742 (issued Aug. 22, 1978); Justice et al., U.S. Pat. No. 4,105,514 (issued Aug. 8, 1978); British Pat. No. 1,448,904 issued to Hooker Chemicals and Plastics Corporation (appication no. 47238/74, filed Oct. 31, 1974); and German OLS's 2,510,396, 2,503,652 and 2,455,222. This increase in voltage, i.e., the increase in cell voltage incurred when operating the cell with the membrane pressurized against the cathode by the anolyte as opposed to the cell voltage incurred when operating the cell with the membrane contacting the anode, will be referred to and defined herein as "voltage penalty".
Nearly all of the above references teach that one of the methods used to maintain the membrane away from the cathode is to maintain the pressure of the catholyte greater than that of the anolyte. Maintaining this pressure difference does urge the membrane away from the cathode and it does reduce cell operational voltage; however it can and does create a hazardous situation when electrolyzing brine for example. Leaks are known to occur in the membrane or around its edges, and when they do under the situation where the catholyte pressure is greater than the anolyte pressure, hydrogen gas generated in the catholyte chamber leaks through or around the membrane into the anolyte chamber where it can easily form an explosive gaseous mixture with the chlorine gas generated in the anolyte chamber. This situation can be avoided by operating the cell with the pressure differences reversed, i.e., with the anolyte pressure greater than the catholyte pressure. For then, if a leak occurs the gas flow is reversed and harmless reaction products are formed. That is when the anolyte pressure is greater than the catholyte pressure and a leak occurs through or around the membrane, then chlorine gas flows through or around the membrane from the anolyte chamber into the catholyte chamber wherein, instead of forming a potentially explosive gaseous mixture with the hydrogen gas, it reacts with the NaOH solution generated therein to form harmless H.sub.2 O, NaCl and NaOCl.
Hence, it can be seen that it would be advantageous to be able to operate such electrolysis cells with the anolyte pressure greater than the catholyte pressure without encountering the high voltage penalties taught as occurring when this is done. It is, therefore, an objective of this invention to discover a type of cathode face against which such membranes can be urged by anolyte pressure greater than the catholyte pressure which do no cause a drastic increase in cell operating voltage.